The present invention relates to electronic measurement and more particularly concerns such measurements that may be readily accommodated to unique characteristics of a measuring probe. Such characteristics include the low gain of low resistance or low output probes and non-linearities of output signals.
Electronic measurement of various conditions, such as temperature, voltage, current, pressure and many others, is commonly accomplished by employing a condition sensitive element, a probe, having an electrical characteristic that varies in accordance with the condition to be measured. Capacitance, magnetic reluctance, inductance, and resistance are among various electrical parameters that are employed in different types of condition sensing probes. Thermocouples and thermistors are often employed in measuring temperature. For temperature measurements over wide ranges, in the order of - 50.degree. Centigrade to + 500.degree. Centigrade, for example, platinum resistance probes are commonly employed. The resistance of a platinum probe varies with temperature. A common probe has a sensitivity or gain or 0.392 ohms per degree Centigrade, within a useful range of temperatures.
Plantinum probes are available in different resistance values, for different gains. A 5000 ohm platinum probe, although it has a higher gain, is less desirable than a probe of lower resistance (a 100 ohm platinum probe) because the higher resistance requires more time to change resistance upon temperature change and is also more costly. The lower resistance probe is easier and cheaper to manufacture and has a time constant of approximately one to two seconds, whereas the 5000 ohm probe is more difficult to fabricate and has a time constant at least several times as great. However, low resistance platinum probes of the prior art provide such a low output (low gain) that even when employing a bridge input, a large amount of costly amplification is required. Generally, such amplification requires the use of operational amplifiers that necessitate two separate power supplies thereby significantly adding to the bulk and weight of the package.
In the use of a platinum temperature responsive probe, resistance of the probe at the temperture to be measured is compared with the finite resistance that the probe exhibits at a reference temperature, such as 0.degree. Centigrade or Farenheit, for example. An arrangement for making such measurement, employing a dual slope technique, designed by George Zively, is described in a Tech Brief of the National Aeronautics and Space Administration, No. B7210545, entitled a Compact Battery Powered Digital Thermometer. In the Zively apparatus, a capacitor is charged through a probe resistance to a level determined by a sample comparator to thereby generate a time interval for pulses that are applied to cause a reversible counter to count up. After completion of this interval, a reference comparator generates a fixed time interval that causes the counter to count down for a fixed time, resulting in a count retained in the counter that is proportional to the difference between the up and down counting interval.
Even with a high resistance probe, gain in the Zively instrument is not high enough and the apparatus must provide counting pulses at frequencies as high as 15 megahertz for adequate resolution of short time intervals provided by the input circuitry. Such frequencies, (required for adequate gain and resolution) are beyond the capabilities of certain commonly available and inexpensive circuitry such as the complimentary metal oxide semi-conductor (C/MOS) devices. Accordingly, adequate gain and resolution, even where high probe resistances are employed, has not been avaialble.
Furthermore, when using a high resistance probe, a relatively high current must be supplied in order to obtain a useful range of temperature sensitivity. The higher current necessarily descreases the time required to charge the timing capacitor to a given percentage of its full charge and accordingly, the full range measurement interval is concomitantly decreased. The short measurement interval requires high frequency clock pulses for adequate resolution. On the other hand, if a longer measuring time interval is available, one can achieve increased accuracy even with a lower clock frequency and smaller probe resistance.
Although the time interval can be increased by increasing capacitance of the timing circuit, there are physical limits on capacitor size in certain types of application. For example, in a hand-held, completely self-contained measuring instrument, the circuitry is made of miniaturized integrated circuit chips and capacitors sufficiently large to provide adequate time constants have too great a physical size to be compatible with the required packaging constraints.
The patent of Krepak U.S. Pat. No. 3,768,310 describes apparatus for temperature measurement in which a thermistor controls the magnitude of current applied to a one shot multivibrator for determination of the time interval thereof. The thermistor has a greater output than a platinum resistance probe and thus Krepak does not need nor does he suggest any way to provide an increased input gain. The arrangement of Krepak, if it employed a platinum probe would also fall to obtain adequate gain and an adequate time interval for miniaturized equipment.
Neither the thermometer of Zively nor that of Krepak show or suggest compensation for probe non-linearity. Although the resistance of a platinum probe will vary in nearly precisely linear manner with temperature from 0.degree. to about 140.degree. Centigrade, the variation exhibits rapidly increasing non-linearity at higher temperatures. This non-linearity has been recognized, as shown and described in the two patents to Dauphinee et al U.S. Pat. No. 2,933,377 and 3,742,764, and the patent to Shimomura U.S. Pat. No. 3,766,782.. The arrangements of Dauphinee et all employ variable resistance networks including a calibrated tapped resistance element having resistor taps available to provide non-linearity compensation at predetermined discrete intervals and with predetermined magnitudes of compensation. Such an arrangement is an approximation at best, and cannot provide accurate compensation for non-linearities between the discrete intervals that have been specifically selected. The arrangement is analagous to a compensation system in which linearized values of a typical probe resistance are calculated for discrete selected temperatures and stored as linearized or compensated values in a memory, commonly a read only memory. A measured resistance value is then employed as an address to extract from the memory the corresponding linearized resistance value. Such an arrangement is only precise for the intervals selected and requires a relatively large amount of storage. For example, if a temperature range of 500.degree. is to be measured in only one-half degree intervals, one thousand measurements must be stored, each requiring four binary bits. Further, the stored information ar the pre-calculated information, as in Dauphinee et al, is accurate only for one specific probe since probe non-linearities may vary from one probe to another. Thus, employing a read only memory or other type of precalculation as in the Dauphinee et al patent, one must either impose rigid tolerances upon characteristics of the probe resistances to be employed or provide a new linearizing memory for each probe.
The patent to Shimomura employs a complex function generator for correction of non-linearity of temperature measurements, producing an inverse function of a variable and replacing the variable by a time function. The correction is available for one sense of measurement, it cannot be readily included and deleted from the circuit, and it operates throughout the entire range of measurement, not just for higher ranges. This complex corrective circuit must have a wide range of high accuracy.
Accordingly, it is an object of the present invention to provide an improved electronic measuring instrument that substantially eliminates or minimizes the above-identified disadvantages.